Field of the Invention
The present invention provides a means of extending the flexibility of a coherent acousto-optic RF spectrum analyzer architecture that can provide rapid and flexible minimization of interfering signals in near real time by using an optical architecture that can provide adaptive filtering, and provides an adaptive wideband acousto-optic RF signal processing system.
Description of the Related Art
Many EW (electronic warfare) and ESM (electronic support measure) systems receive and attempt to process wide bandwidth portions of the RF spectrum seeking to capture Signals of Interest (SOI). Often, these SOI have wide instantaneous bandwidth parameters, such as spread spectrum radar and communications signals, but many are also frequency agile, rapidly changing their RF frequency parameters. It is also common for EW and ESM systems to encounter interfering signals that obscure the parameters of the SOI. Often these interfering signals are intentionally generated to obscure the SOI, and often these interfering signals are themselves complex wideband signals of rapidly changing parameters.
A common method of reducing the effects of interfering signals is to employ an adaptive filter to block out interfering signals from the received RF spectrum being processed by the EW or ESM system. The adaptive filter may use frequency, Angle of Arrival (AOA), or Time of Arrival (TOA) information to develop the adaptive filtering parameters. In the simplest case, the interfering signal is a CW emitter that can simply be minimized by a narrow bandwidth band-reject “notch” filter, often performed by a Least Means Squared (LMS) algorithm in an accompanying DSP (Digital Signal Processing) signal processing subsystem in the EW or ESM system. In a dense signal environment, e.g., the European theater at 30,000 feet with a typical EW or ESM Field of View (FOV), the number of interfering signals increases dramatically. Multiple “notch” filters within the same spectrum sample become increasingly complex and difficult to create without causing additional intermodulation interfering (false) signals and accompanying distortion of the SOI. Wide bandwidth and complex interfering signals are also difficult to minimize since they have multiple frequency components within the RF spectrum of interest, and may be agile, requiring the adaptive minimization of interfering signals.
It is known that a coherent acousto-optic RF spectrum analyzer provides a wide instantaneous bandwidth Fourier Transform image of an input RF spectrum, i.e., the acousto-optic Bragg cell serves as a spatial light modulator (SLM) that accepts a wideband RF signal containing all signals within the RF spectrum of interest, and creates a spatially distributed one-dimensional optical image of the RF spectrum. Bragg cell based acoustic optic spectrum analyzers achieving more than 1 GHz instantaneous RF bandwidth and greater than 40 dB SFDR (Spurious Free Dynamic Range) have been demonstrated, and provide enough signal processing performance to be useful in EW and ESM applications. The acousto-optic SLM diffracts an incoming LASER beam into multiple first order diffracted beams, each first order diffracted beam corresponding to an RF signal present in the RF input spectrum. A Fourier lens is located one focal length from the Bragg cell, in the object plane to capture the first order diffracted beams, and then performs the complex Fourier Transform of the beams to create a one-dimensional line image of the frequency domain (RF spectrum) at the image plane located one focal length behind the Fourier lens. Past constructions of this architecture used multiple photodetectors (“linear arrays”) arranged along this one-dimensional line image to detect the optical signals and transduce them back to the original RF signals, with each photodetector capturing a portion of the RF spectrum determined by the spatial width dimensions of the photodetector's active area. This architecture provides a means for creating a “channelized receiver” that can minimize interfering signals by choosing to ignore the signal outputs from those photodetectors that represent interfering signals. However, the “channelized received” lacks the flexibility to adapt to varying interfering signal bandwidths and more complex wide bandwidth signals that often occupy more than one photodetector's portion of the spectrum, as the optics and the photodetector dimensions are fixed variables and therefore not conducive to adaptive requirements.